1. Field of the Invention
The present invention relates to active clamping circuits, and more particularly, to an active clamping circuit applicable to a DC-to-DC conversion circuit.
2. Description of the Related Art
FIGS. 1a and 1b show a conventional step-down (buck) DC-to-DC conversion circuit 10. In the DC-to-DC conversion circuit 10 is coupled to an input voltage Vin and outputs an output voltage VCORE to a load LD. In the DC-to-DC conversion circuit 10, the switching devices S1 and S2 are switched alternately to maintain the output voltage VCORE at a predetermined voltage, such as 1.35V. When the output voltage VCORE is lower than the predetermined voltage, the switching device S1 is turned on and the switching device S2 is turned off such that the input voltage Vin charges the capacitor Co to pull high the output voltage VCORE. On the contrary, when the output voltage VCORE is higher than the predetermined voltage, the switching device S1 is turned off and the switching device is turned on such that the input voltage Vin stops charging the capacitor Co.
Transient responses, however, typically occur when the load LD varies greatly, such as when the CPU is switched to suspend mode from normal mode or when the CPU is switched to suspend mode from suspend mode. As shown in FIG. 1a, for example, when the CPU is switched to normal mode from suspend mode, the load LD is transferred from a light to a heavy load such that the output voltage VCORE is instantly pulled low. Thus, the switching device S1 is turned on and the switching device S2 is turned off such that the capacitor Co is charged by the input voltage Vin to pull high the output voltage VCORE. The rate of change in the inductor current iL is
            ⅆ      iL              ⅆ      t        =                    Vin        -                  V          CORE                    L        .  As shown in FIG. 1b, when the CPU is switched to suspend mode from normal mode, namely the load LD is transferred from a heavy to a light load such that the output voltage VCORE is instantly pulled high. Thus, the output voltage is instantly pulled high such that the switching device S1 is turned off and the switching device S2 is turned on to discharge the output voltage VCORE to the predetermined voltage by the switching device S2. The rate of change in the inductor current iL is
            ⅆ      iL              ⅆ      t        =                    -                  V          CORE                    L        .  
When the load LD is transferred from a heavy to a light load (step-down period), however, the current rate is much smaller than that from light to heavy (step-up period) because the output voltage VCORE is much higher than the input voltage Vin in the conversion circuit 10. That is, the conversion circuit 10 has poor transient responses when load LD is transferred from a heavy to a light load (step-down period).